Audio amplifiers, especially digital audio amplifiers, are more and more popular in consumer electronics such as TV sets, automobile audios and DVD players. Conventionally, a pulse width modulated (PWM) input power stage may be configured as an output stage of a digital audio amplifier. However, traditional PWM input power stages typically have an open-loop configuration. FIG. 1 schematically illustrates a conventional single-ended audio amplifier 100 that includes an output power stage 101 configured to receive a PWM input signal and provide a square-wave signal; a filtering stage 102 configured to receive the square-wave signal and generate an output signal to drive a load 103 such as a speaker.
In FIG. 1, the output power stage 101 is in an half-bridge configuration, and comprises a first switching device M1, a second switching device M2 and a driving circuit 104. The first switching device M1 and the second switching device M2 are coupled in series between an input power supply Vcc and ground, and the driving circuit 104 is configured to receive the PWM input signal and to generate gate driving signals to drive the first switching device M1 and the second switching device M2.
M1 and M2 are turned on and off periodically in a complementary manner such that a square-wave signal ASW is provided at a common node SW of the switches M1 and M2. The square-wave signal ASW is subsequently filtered by the filtering stage 102 and an output signal OUT1 is generated and provided to the speaker 103. In the audio amplifier 100 shown in FIG. 1, the filtering stage 102 comprises a low-pass filtering inductor L1, a low-pass filtering capacitor C1 and an output DC-blocking capacitor COUT1. Since the output power stage 101 is in an open-loop configuration, the square-wave signal ASW may have a duty cycle that changes based on a duty cycle of the PWM input signal. When the input power supply voltage Vcc changes, the output signal OUT1 may change accordingly. The gain of the single-ended audio amplifier 100 may vary with the input power supply voltage Vcc. Thus, the single-ended audio amplifier 100 may have a poor power supply rejection performance and a low power supply rejection ratio (“PSRR”).
For an audio amplifier with a bridge tied load (“BTL”) that is generally configured based on two output power stages in half-bridge and open loop configurations, with zero input, the BTL audio amplifier may have differential output signals. Therefore, output voltage ripples in the differential output signals caused by a variation in a power supply voltage of the BTL audio amplifier may be counteracted. Thus, the BTL audio amplifier with an open loop configuration may have a good power supply rejection performance. However, a BTL audio amplifier with open loop configuration may not be able to provide a stable output voltage during a transient phase of a power supply voltage of the BTL audio amplifier.